http://hackage.haskell.org/package/base-4.9.1.0/docs/Debug-Trace.html#v:trac...
My sense is that you should probably be using traceIO rather than the
pure trace functions. The pure functions do not sequence with respect
to IO and once the expression you are tracing has been fully
evaluated, they will never take effect again.
On Wed, Jun 28, 2017 at 11:16 AM, Jurriaan Hage
We had a strange phenomenon while debugging a large compiler. This is the smallest example, but we have seen this behavior in our own monad.
module TracEx where
import Debug.Trace
runit :: IO () runit = traceShow "Bla" $ do traceShow "Bloe” runit
and this generates Bla Bloe
and then it hangs. Nothing else is generated. We are guessing this has to do with optimisations of some kind.
And what about
runit :: IO () runit = traceShow "Bla" $ do n <- traceShow "Num" print 2 traceShow "Bloe” runit
Now we get Bla Num 2 Bloe 2 Bloe 2 … No more Bla and no more Num.
Any ideas whether this is a bug or a feature?
Jur and Tibor
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